In recent years, poles for holding street lights outdoors have been made taller and taller. Since they are further from the ground, they employ more powerful lights and appropriate lenses for dispersing the light so that the lighted area is far greater than with lower poles. The rationale for using fewer, very tall poles rather than a larger number of shorter poles adjacent the highway, especially at highway intersections, is that fewer poles reduce the accident hazard to motorists.
The newer poles have attained heights of about 150 feet. With such height, their bulbs or lamps are out of the range of conventional "cherry pickers" and therefore servicing of the lights including bulb replacement is more difficult than with shorter poles. Nor is it practical to have built-in or removable ladders to enable maintenance personnel to ascend the pole to service the luminaries.
To meet the more difficult problems of bulb replacement and general maintenance, a number of schemes have been evolved. At first, the individual lights were suspended by steel cables which passed the hollow pole and over pulleys at the top so that each light could be lowered to the ground when desired simply by unwinding a sufficient length of cable. The problem with this arrangement was that lights that simply hung from individual steel cables tended to be blown around or to sway voilently in the wind causing cable wear and damage to the lights by the continuous movement. Also, over an extended period of time, the steel cables tended to stretch causing the lights to be lower than their normal height.
A later approach was to mount several of the lights on a movable carrier which was lowered to the ground by internal steel cables within the pole to permit access to the lights and then was raised by the same cable to normal height. To relieve some of the constant strain that would be imposed on the cables if they were used as the sole support of the carrier in elevated position, the carrier included a number of latch mechanisms which engaged corresponding mechanisms on the mast head at the top of the pole. Even with the use of the light carrier, the construction of the newer poles with such luminaire raising and lowering systems continued to present safety problems. First of all, the carrier itself could weigh 1500-1800 pounds so that, in the event of a mishap during raising or lowering it, if it fell, it could endanger the life of the maintenance worker or others below.
Second, even where a number of steel cables have been used to lower the carrier, the cables often become stretched and different cables may have different stretching characteristics. This stretching could even be aggravated by unevenness in the raising of the carrier which, in turn, itself contributes to additional stretching.
Third, the internal cable running up to the carrier can sometimes run off the sides of the winch reel causing "down time" of the luminaire because the cable can be damaged and has to be replaced. In some instances, it might even be dangerous to unwind severely damaged cable so that the entire pole might have to be taken down by a crane in order to change the cable or correct the problem.
It is therefore among the objects of the present invention to provide:
1. Tall luminaire poles with safer, more stable lowering carriers.
2. Tall luminaire poles with built-in systems for detecting unsafe conditions, especially conditions resulting from stretched, slack or stuck lowering cables.
3. Tall luminaire poles with improved means for latching the light carrier to the top of the pole.
4. Tall luminaire poles with means for compensating for unequal stretch or length of the cables attached to the carrier.
5. Tall luminaire poles with means for compensating for unevenness in the raising (or lowering) of the carrier.